Flowmeter calibration techniques

ABSTRACT

Descriptions are provided for implementing flowmeter zeroing techniques. In operating a flowmeter, it may be the case that, if not properly calibrated, the flowmeter will produce erroneous measurements, e.g., will indicate a non-zero flow during a period of zero flow. By determining a magnitude of such erroneous measurements, calibration values may be determined, which may later be used to adjust a measurement that is output by the flowmeter and thereby improve an accuracy of the flowmeter. Such calibration values may be determined for a plurality of operational conditions associated with the flowmeter, such as densities of materials being measured, and/or configurations of flow elements associated with transporting material to the flowmeter. Then, the calibration values may be correlated with the relevant operational conditions, and stored for later use. In this way, during an actual operation of the flowmeter, a number of calibration values may be made available, and an optimal calibration value may be selected for an existing operational condition of the flowmeter.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/729,990, Dec. 9, 2003, titled “FLOWMETER ZEROING TECHNIQUES” whichclaims priority to U.S. Provisional Application Ser. No. 60/460,817,filed on Apr. 8, 2003, and titled “FLOWMETER ZEROING TECHNIQUES.” Bothof these applications are incorporated herein in their entirety.

TECHNICAL FIELD

This description relates to flowmeters.

BACKGROUND

Flowmeters provide information about materials being transferred througha conduit. For example, mass flowmeters provide a measurement of themass of material being transferred through a conduit. Similarly, densityflowmeters, or densitometers, provide a measurement of the density ofmaterial flowing through a conduit. Mass flowmeters also may provide ameasurement of the density of the material.

For example, Coriolis-type mass flowmeters are based on the Corioliseffect, in which material flowing through a conduit becomes aradially-travelling mass that is affected by a Coriolis force andtherefore experiences an acceleration. Many Coriolis-type massflowmeters induce a Coriolis force by sinusoidally oscillating a conduitabout a pivot axis orthogonal to the length of the conduit. In such massflowmeters, the Coriolis reaction force experienced by the travelingfluid mass is transferred to the conduit itself and is manifested as adeflection or offset of the conduit in the direction of the Coriolisforce vector in the plane of rotation.

SUMMARY

According to one general aspect, a flowmeter is calibrated. A pluralityof calibration values are determined, the calibration valuescorresponding to measurements of material in a flowtube, the flowtubebeing associated with the flowmeter. Each of the calibration values isassociated with one of a plurality of operational parameters of theflowmeter, each of the operational parameters being present during thedetermining of its corresponding calibration value. The calibrationvalues are stored in association with their respective operationalparameters.

Implementations may have one or more of the following features. Forexample, in determining the plurality of calibration values a pluralityof zero-flow calibration values corresponding to the measurements may bedetermined, where the measurements include mass flow measurementserroneously indicated by the flowmeter during a time of substantiallyzero mass flow through the flowtube. A current operational parameter ofthe flowmeter may be determined, and a current calibration value for useduring an obtaining of a mass flow measurement may be determined, basedon the current operational parameter.

In determining the current operational parameter, a density of thematerial in the flowtube may be determined. In this case, determiningthe current calibration value of the flowmeter may include measuring acurrent density of a current material in the flowtube.

In associating each of the calibration values with one of the pluralityof operational parameters, a first calibration value may be associatedwith a range of densities. In this case, in determining the currentcalibration value, a current density of a current material in theflowtube may be measured, it may be determined that the current densityfalls within the range of densities, and the first calibration value maybe selected.

In associating each of the calibration values with one of the pluralityof operational parameters, a mathematical relationship between thecalibration values and the plurality of operational parameters may beused. In this case, in determining the current calibration value, acurrent density of a current material in the flowtube may be measured,the current density may be used in conjunction with the mathematicalrelationship to determine a current calibration value, and the currentcalibration value may be selected.

In determining the current operational parameter, a configuration offlow elements associated with the flowtube may be determined. In thiscase, in determining the current calibration value, an input may beaccepted from a user, the input identifying a current configuration offlow elements. Additionally, or alternatively, in determining thecurrent calibration value, a first density of a first material in theflowtube may be measured, a correlation between the first density and afirst configuration of flow elements may be accessed, and a firstcalibration value corresponding to the first configuration may beselected.

In determining the current operational parameter, a gas void fraction ofthe material in the flowtube may be determined. In this case, indetermining the current calibration value, an input of the gas voidfraction may be received from a gas void fraction measurement system,and the current calibration value may be selected from a pre-determinedlist of associated calibration values and gas void fractions.

According to another general aspect, a calibration system includes ameasurement system operable to output measurements of material in aflowtube, where the flowtube is associated with a flowmeter, acalibration system operable to determine calibration values, each basedon a corresponding measurement output by the measurement system, and amemory operable to store each of the calibration values in conjunctionwith an operational parameter associated with an operation of theflowmeter at a time of the corresponding measurement.

Implementations may have one or more of the following features. Forexample, the calibration system may be operable to select a calibrationvalue from the memory, based on a current operational parameterassociated with the flowmeter. The measurement may include a mass flowrate of the material, and further wherein the calibration values mayinclude zero calibration values corresponding to erroneously-detectedmass flow measurements of the material during a time of substantiallyzero flow.

The operational parameter may include a density of the material. Thecalibration system may be. operable to select a current calibrationvalue based on a current density of material in the flowtube, asmeasured by the measurement system. The calibration system may beoperable to select the current calibration value by associating thecurrent density with a pre-selected range of densities that is stored inthe memory in association with the current calibration value. Thecalibration system may be operable to select the current calibrationvalue, based on the current density and a mathematical relationshipbetween the calibration values and their corresponding operationalparameters The operational parameter may include a configuration of flowelements associated with the flowtube. The calibration system may beoperable to accept a current configuration input by an operator, and toselect a current calibration value based on the current configuration.The calibration system may be operable to select a current calibrationvalue by determining a current configuration based on acurrently-measured density that was previously associated with thecurrent configuration.

The operational parameter may include a gas void fraction of flowelements within the flowtube. The calibration system may be operable toreceive a current gas void fraction from a gas void fraction measurementsystem, and further may be operable to select a corresponding currentcalibration value from the memory.

The measurement system, the calibration system, and the memory may beintegrated with the flowmeter.

According to another general aspect, a flowmeter is operated. Anoperational parameter associated with the flowmeter is determined. Azero-flow calibration value based on the operational parameter isdetermined, based on a plurality of previously-determined zero-flowcalibration values. A measurement of a property of a material within aflowtube associated with the flowmeter is taken, using the flowmeter.The measurement is adjusted using the zero-flow calibration value.

Implementations may have one or more of the following features. Forexample, in determining the operational parameter, determining aconfiguration of flow elements associated with the flowtube may bedetermined. In determining the zero-flow calibration value, thezero-flow calibration value may be selected from among thepreviously-determined zero-flow calibration values as being thezero-flow calibration value that corresponds to one of a set ofconfigurations, where each of the set of configurations existed at atime when its corresponding zero-flow calibration value was previouslydetermined.

In determining the operational parameter, a selection of theconfiguration may be accepted from a pre-determined set ofconfigurations. Additionally, or alternatively, a density of thematerial may be measured, and the density may be associated with a firstconfiguration.

The operational parameter may include a density of the material in theflowtube. In this case, determining the zero-flow calibration value mayinclude associating the density with a range of densities, and selectingthe zero-flow calibration value from among the plurality ofpreviously-determined zero-flow calibration values, based on apre-determined relationship between the range of densities and thezero-flow calibration value.

In determining the zero-flow calibration value, the density may be inputinto a mathematical relationship derived from a relationship between thepreviously-determined zero-flow calibration values and correspondingdensity measurements. Additionally, or alternatively, the zero-flowcalibration value may be selected from among the plurality ofpreviously-determined zero-flow calibration values, based on apre-determined relationship between the operational parameter and thezero-flow calibration value.

In determining the operational parameter, a gas void fraction of thematerial in the flowtube may be determined. In determining the gas voidfraction, a current gas void fraction may be received from a gas voidfraction measurement system, and further wherein determining thezero-flow calibration value comprises selecting a current zero-flowcalibration value previously associated with the current gas voidfraction measurement.

The details of one or more implementations are set forth in theaccompanying drawings and the description below. Other features will beapparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1A is an illustration of a Coriolis flowmeter using a bentflowtube.

FIG. 1B is an illustration of a Coriolis flowmeter using a straightflowtube.

FIG. 2 is a block diagram of a Coriolis flowmeter.

FIG. 3 is a block diagram of a flowmeter operating in conjunction withone or more of a plurality of different-density materials.

FIG. 4 is a chart comparing zero calibration values calculated atdifferent fluid densities.

FIG. 5 is a block diagram of a flowmeter operating in conjunction with aplurality of piping and/or pumping configurations.

FIG. 6 is a flowchart illustrating techniques for selecting a zerocalibration value.

DETAILED DESCRIPTION

Types of flowmeters include digital flowmeters. For example, U.S. Pat.No. 6,311,136, which is hereby incorporated by reference, discloses theuse of a digital flowmeter and related technology including signalprocessing and measurement techniques. Such digital flowmeters may bevery precise in their measurements, with little or negligible noise, andmay be capable of enabling a wide range of positive and negative gainsat the driver circuitry for driving the conduit. Such digital flowmetersare thus advantageous in a variety of settings. For example,commonly-assigned U.S. Pat. No. 6,505,519, which is incorporated byreference, discloses the use of a wide gain range, and/or the use ofnegative gain, to prevent stalling and to more accurately exercisecontrol of the flowtube, even during difficult conditions such astwo-phase flow.

Although digital flowmeters are specifically discussed below withrespect to FIGS. 1 and 2, it should be understood that analog flowmetersalso exist. Although such analog flowmeters may be prone to typicalshortcomings of analog circuitry, e.g., low precision and high noisemeasurements relative to digital flowmeters, they also may be compatiblewith the various techniques and implementations discussed herein. Thus,in the following discussion, the term “flowmeter” or “meter” is used torefer to any type of device and/or system in which a Coriolis flowmetersystem uses various control systems and related elements to measure amass flow, density, and/or other parameters of a material(s) movingthrough a flowtube or other conduit.

FIG. 1A is an illustration of a digital flowmeter using a bent flowtube102. Specifically, the bent flowtube 102 may be used to measure one ormore physical characteristics of, for example, a (traveling) fluid, asreferred to above. In FIG. 1A, a digital transmitter 104 exchangessensor and drive signals with the bent flowtube 102, so as to both sensean oscillation of the bent flowtube 102, and to drive the oscillation ofthe bent flowtube 102 accordingly. By quickly and accurately determiningthe sensor and drive signals, the digital transmitter 104, as referredto above, provides for fast and accurate operation of the bent flowtube102. Examples of the digital transmitter 104 being used with a bentflowtube are provided in, for example, commonly-assigned U.S. Pat. No.6,311,136.

FIG. 1B is an illustration of a digital flowmeter using a straightflowtube 106. More specifically, in FIG. 1B, the straight flowtube 106interacts with the digital transmitter 104. Such a straight flowtubeoperates similarly to the bent flowtube 102 on a conceptual level, andhas various advantages/disadvantages relative to the bent flowtube 102.For example, the straight flowtube 106 may be easier to (completely)fill and empty than the bent flowtube 102, simply due to the geometry ofits construction. In operation, the bent flowtube 102 may operate at afrequency of, for example, 50-110 Hz, while the straight flowtube 106may operate at a frequency of, for example, 300-1,000 Hz.

Referring to FIG. 2, a digital mass flowmeter 200 includes the digitaltransmitter 104, one or more motion sensors 205, one or more drivers210, a flowtube 215 (which also may be referred to as a conduit, andwhich may represent either the bent flowtube 102, the straight flowtube106, or some other type of flowtube), and a temperature sensor 220. Thedigital transmitter 104 may be implemented using one or more of, forexample, a processor, a Digital Signal Processor (DSP), afield-programmable gate array (FPGA), an ASIC, other programmable logicor gate arrays, or programmable logic with a processor core.

The digital transmitter 104 generates a measurement of, for example,density and/or mass flow of a material flowing through the flowtube 215,based at least on signals received from the motion sensors 205. Thedigital transmitter 104 also controls the drivers 210 to induce motionin the flowtube 215. This motion is sensed by the motion sensors 205.

Density measurements of the material flowing through the flowtube arerelated to, for example, the frequency of the motion of the flowtube 215that is induced in the flowtube 215 by a driving force supplied by thedrivers 210, and/or to the temperature of the flowtube 215. Similarly,mass flow through the flowtube 215 is related to the phase and frequencyof the motion of the flowtube 215, as well as to the temperature of theflowtube 215.

The temperature in the flowtube 215, which is measured using thetemperature sensor 220, affects certain properties of the flowtube, suchas its stiffness and dimensions. The digital transmitter 104 maycompensate for these temperature effects. This temperature affects, forexample, an operating frequency of the digital transmitter 104, asampling rate of an analog-to-digital converter, and/or a crystalfrequency associated with a reference clock used by the transmitter 104.Also in FIG. 2, a gas void fraction sensor 222 (discussed in more detailbelow) is included that is operable to determine what percentage of amaterial in the flowtube 215, if any, is composed of a gas. Although notshown in FIG. 2, other sensors may be included, such as, for example, apressure sensor that is operable to sense a pressure of a materialflowing through the flowtube 215.

In performing measurements of mass flow (and/or density), calibration ofthe flowmeter may be required in order to maintain system performance,particularly over long periods of time and/or extensive operation of theflowmeter. One calibration technique is known as “zeroing” or “a zerocalibration.” In a zeroing process, a flow of material through theflowtube 215 is stopped (for example, upstream and downstream valves maybe closed) during a time when the flowtube 215 is filled with thematerial. As a result, there is a zero flow of the material, whichshould lead to a corresponding reading of zero flow output by theflowmeter.

For various reasons, however, it may be the case that the flowmeteroutputs a non-zero (i.e., erroneous) flowrate during a time of zeroflow. In these cases, the (erroneous) non-zero flowrate at zero flow maybe used as a calibration factor; that is, for example, it may besubtracted from (or added to) a measured flow, so as to obtain anaccurate, i.e., zero-corrected, reading of the flow during futurereadings.

The zeroing process might involve, for example, filling the flowtube 215with a process fluid. Then the flow is brought to zero while keeping themeter full. The resulting non-zero, erroneous flow indicated by theflowmeter may be averaged over a period of time to determine the zerocalibration value.

In performing such a zeroing process, a type of flowtube currently beingused may be relevant in obtaining and using the zero calibration value.Additionally, it may be the case that the zero calibration value also isa function of factors or parameters that are related to the particularoperation of whatever flowmeter being used. That is to say, the zerocalibration value may be a function of various parameters, such as, forexample, a density of the process fluid(s), a configuration of externalelements associated with the flowmeter (e.g., valves, pumps, pipes), ora temperature (as measured by the temperature sensor 220). As a result,when one or more of these parameters changes, the previously-calculatedzero calibration value may be less accurate.

For example, in a Coriolis mass flowmeter, the zero calibration valuemay be different for fluids having different densities. One reason forthis may be that the drive frequency for the flowtube 215 is selected inpart based on fluid density, and the mass flow reading at zero flow maybe largely dependent on frequency. Thus, in applications where fluids ofgreatly varying density are involved, or applications where multiplefluids are passed through the same meter, the calculated zerocalibration value may not be sufficiently accurate.

As another example, the phrase “batching from empty” refers to acondition in which a flowmeter is completely emptied in between batchesof measured materials (i.e., is empty at a start and finish of themeasurements). In this situation, the zero calibration value when theflowtube 215 is full may be considerably different than the zerocalibration value when the flowtube is empty. Other examples in which azero calibration value may require changing in response to changed (orchanging) flowmeter conditions are discussed in more detail below.

In order to provide an accurate zero calibration value in a wide varietyof settings, separate zero calibration values are determined indifferent environments, and correlated with the environment in whichthey were obtained. Then, during subsequent use of the flowmeter, anappropriate zero calibration value is determined for the environment athand, based on the previously-obtained values.

In FIG. 2, then, the digital transmitter 104 includes various elementsdesigned to implement the above-described calibration functionality. Itshould be understood that some or all of these elements (along withother elements of the digital transmitter 104, not explicitly shown inFIG. 2) also may be used during a normal operation of the flowmeter 200(e.g., after zero calibration values have been determined).

Thus, the digital transmitter 104 includes a measurement system 225,which is operable to obtain a measurement, such as a mass flowmeasurement and/or density measurement. A calibration determination andcorrelation system 230, based on at least one measurement output by themeasurement system 225, determines a zero calibration valuecorresponding to an operational parameter of the flowmeter 200, at thetime of the measurement. For example, the measurement system 225 mayoutput an erroneous, non-zero measurement of mass flow of a givenmaterial during a time of substantially zero flow of the materialthrough the flowtube 215, as well as a density of the material. Thecalibration determination and correlation system 230 may then determinea zero calibration value associated with the mass flow measurement andcorrelate that zero calibration value with the measured density, to beused as a correction factor during future mass flow measurements ofmaterial(s) having that density.

Thus, as in the example just given, the operational parameter mayinclude, for example, a parameter that is detectable and measurable bythe measurement system 225, such as the density (or temperature) of thematerial. As another example, the operational parameter may include aparameter that may be input into the calibration determination andcorrelation system 230 by an operator, such as a configuration of theflowmeter, or a configuration of various flow elements associated withthe flowmeter, such as pipes, valves, or manifolds.

The calibration determination and correlation system 230 correlatesparticular zero calibration values with particular, correspondingoperational parameters, and stores these values and correlations in amemory 235. During a subsequent operation of the flowmeter 200, acalibration selection system 240 may select or otherwise determine anappropriate zero calibration value, using the memory 235. In oneimplementation, the calibration system determines the appropriate zerocalibration value automatically, whereas, in another implementation, anoperator selects the appropriate zero calibration value directly fromthe memory 235. Thus, an appropriate zero calibration value is madeavailable to the measurement system 225, so that the measurement system225, during normal operation of the flowmeter 200, may outputhighly-accurate, zero-adjusted measurements.

The calibration selection system 240 may select a zero calibration valuefrom the memory 235 in a variety of ways. For example, when zerocalibration values and operational parameters that include acorresponding number of fluid densities are stored in the memory 235,the calibration selection system 240 may interact with the measurementsystem 225 to determine a density of a material currently in theflowtube 215. Then, the calibration selection system may select a zerocalibration value corresponding to that density. In this way,highly-accurate, customized mass flow measurements may be obtainedduring future use of the flowmeter, even when a plurality ofdifferent-density materials are measured by the flowmeter.

FIG. 3 is a block diagram 300 of a flowmeter operating in conjunctionwith one or more of a plurality of different-density materials. In FIG.3, a liquid material is fed into a valve 302, and thereby into a meter304. Further in FIG. 3, a vapor or gas material is fed through a valve306 and into the meter 304. It should be understood that, although notexplicitly shown in FIG. 3, the meter 304 may include, for example, allof the features described above with respect to the flowmeter 200 andfurther described in U.S. Pat. Nos. 6,311,136 and/or 6,505,519.

In FIG. 3, the valve 302 may be opened while the valve 306 is closed, sothat the meter 304 measures only the liquid material. Conversely, thevalve 306 may be opened while the valve 302 is closed, so that the meter304 measures only the gaseous material. Further, the valves 302 and 306may each be partially or alternately open, so that both of the liquidand gaseous materials travel through the meter 304 during a given periodof time.

Thus, as described above, a density of the liquid and a density of thegas (e.g., air) may be associated with an appropriate zero calibrationvalue. Then, during subsequent measurements, the meter 304 may determinea density of the material currently flowing therethrough, and select thecorresponding zero calibration value accordingly.

It should be understood that many variations exist for the configurationshown in FIG. 3. For example, more than two materials could be measuredin this way by the meter 304. Also, multiple types of liquids (orgasses) could simultaneously be included (as opposed to one or more ofeach).

In one implementation, a mixture of the same two fluids is passedthrough the meter 304, where proportions of the fluids relative to oneanother may vary. For example, in FIG. 3, the liquid and the gas may bepassed through the meter 304 simultaneously, rather than consecutively,and the percentage of the gas (i.e., the “void fraction”) may vary overtime. As the void fraction increases, the density of the liquid/gasmixture will generally decrease, possibly resulting in the need for adifferent zero calibration value.

Various techniques exist for measuring the gas void fraction. Forexample, various sensors or probes exist that may be inserted into theflow to determine a gas void fraction. As another example, a venturitube (i.e., a tube with a constricted throat that determines fluidpressures and velocities by measurement of differential pressuresgenerated at the throat as a fluid traverses the tube), relying on thefact that gas generally moves with a higher velocity than liquid(s)through a restriction, may be used to determine a pressure gradient andthereby allow a determination of the gas void fraction. In some systems,as referred to above with respect to FIG. 2, measurements of gas voidfractions may be obtained using equipment (e.g., the gas void fractionsensor 222) that is wholly external to the flowtube. For example, sonarmeasurements may be taken to determine gas void fraction. As a specificexample of such a sonar-based system, the SONARtrac™ gas void fractionmonitoring system produced by CiDRA Corporation of Wallingford, Conn.may be used.

The situation of a void fraction arises, for example, when the meter 304measures a liquid and gas flow simultaneously (e.g., when measuring acombination of oil and hydrocarbon-related gas that is output by an oilwell), or when a meter is used in a “batching from empty” application,as referred to above. In these and similar situations, the user might,for example, zero the meter 304 while full of liquid, and store the zerocalibration value and density. This process may be repeated with themeter 304 being empty (e.g. full of air). Then, during operation, themeter 304 would use the zero calibration value associated with gas whenit is predominantly gas and would use the zero calibration valueassociated with liquid when it is closer to being full. This processcould be further extended to store zero calibration values for a varietyof void fractions.

Additionally, or alternatively, any of the various techniques discussedabove for determining gas void fractions (or any other technique) may beused to determine gas void fractions, which may then be associated withcorresponding zero calibration values. In one implementation, a gas voidfraction measurement system, such as, for example, an external sensor orprobe similar to those discussed above, may output a signal directly toan input of the transmitter 104, to be processed in the transmitter 104to determine a gas void fraction (i.e., may be positioned and connectedsimilarly to the temperature sensor 220 of FIG. 2). In anotherimplementation, the signal from such an external sensor or probe may beconnected to a separate computing resource, such as, for example, acontrol system or flow computer, with the determination of a gas voidfraction carried out therein. In yet another implementation, the sensoror probe itself may have an integrally-formed computing resource that isoperable to determine a gas void fraction for direct output to thetransmitter 104.

FIG. 4 is a chart 400 comparing zero calibration values calculated atdifferent fluid densities. In the chart of FIG. 4, a first data point402 corresponds to a zero calibration value determined (by thecalibration determination and correlation system 230) for a materialhaving a first density, as measured by the measurement system 225.Similarly, second and third data points 404 and 406 correspond to zerocalibration values determined for materials having second and thirddensities, respectively. The data points 402, 404, and 406 may then beassociated with their respective density values by the calibrationdetermination and correlation system 230, and stored in the memory 225,so that an appropriate zero calibration value may be chosen by thecalibration selection system 240 during future measurements.

For example, similarly to the implementation of FIG. 3, it may be thecase that (in this case, three) different fluids are alternatelyprocessed through the same flowmeter, where each of the three differentfluids has a density corresponding to one of the data points 402, 404,and 406. In one implementation, then, the calibration selection system240 might simply select a zero calibration value corresponding to ameasured one of the three densities, perhaps from a table relating thezero calibration and density values and stored in the memory 235.

In another example, a fluid may be used having a density that does notdirectly correspond to one of the data points 402, 404, or 406. In thiscase, the three data points 402, 404, and 406 may nonetheless be used toobtain an appropriate zero calibration value. For example, a number ofdensity bands 408, 410, 412, 414, 416, and 418 might be defined within arelevant density range, so that the zero calibration valuescorresponding to the data points 402, 404, and 406 may be associatedwith a corresponding one of the density bands. The bands may beestablished in the calibration determination and correlation system 230and/or the calibration selection system 240, and stored in the memory235. The bands may be based on, for example, a manufacturer'sdetermination, or could be a configurable parameter that is adjusted bya user of the flowmeter.

Once the bands are established for a fluid having a density within oneof the bands, an appropriate zero calibration value may be chosen. Forexample, if a fluid having a density falling within the density band 410is processed by the flowmeter, then the zero calibration valuecorresponding to the data point 402 may be used.

Somewhat similarly, if the data points 402, 404, and 406 have some typeof relationship to one another, such as a linear, quasi-linear, ortractable relationship, the calibration determination and correlationsystem 230 may interpolate (e.g., linearly interpolate) between the datapoints. In this way, a fluid having a density anywhere within therelevant range could be correlated with a corresponding zero calibrationvalue that would produce an improved-accuracy measurement. In this case,the memory 235 may be used to store a mathematical relationship oralgorithm derived by the calibration determination and correlationsystem 230, rather than (or in addition to) a table of related values.

FIG. 5 is a block diagram 500 of a flowmeter 502 operating inconjunction with a plurality of piping and/or pumping configurations. InFIG. 5, a first material or product 504 is pumped through a pump 506 anda valve 508 before reaching the meter 502. A second product 510 ispumped through a pump 512 and a valve 514 before reaching the meter 502.Finally in FIG. 5, a third product 516 is sent to the meter 502 afterbeing fed to a valve 518 by way of a gravity feed 520. Thus, the firstproduct 504, the pump 506, and the valve 508 form a first configuration522, the second product 510, the pump 512, and the valve 514 form asecond configuration 524, and the third product 516, valve 518, and thegravity feed 520 form a third configuration 526.

In FIG. 5, the three products 504, 510, and 516 may or may not have thesame density as one another. Even in the case where two or more of thedensities are the same (or even where two of the products are the same),however, zero calibration values associated with each of theconfigurations 522, 524, and 526 may be different from one another, dueto, for example, the configuration elements (e.g., the valves 508, 514,518, the pumps 506 and 512, or the gravity feed 520) that are used,and/or the manner in which the configuration elements are connected toone another.

Thus, in one exemplary case, and similar to the chart of FIG. 4, a zerocalibration value is determined for (and associated with) each of theconfigurations 522, 524, and 526, and stored for future use. Selectionof an appropriate zero calibration value is then determined duringmeasurements by way of discrete input from an operator of the flowmeter502, e.g., by input into the calibration determination and correlationsystem 230 and/or the calibration selection system 240 of acurrently-used configuration.

In a more specific example, the various zero calibration values areassociated with their respective configurations in the above-describedmanner, and the configurations and corresponding zero calibration valuesare then displayed on an output screen associated with the digitaltransmitter 104. In this way, a user may simply scroll through thestored configurations to select an appropriate zero calibration value.

In a further example, if the three products have different densities,then these densities are correlated with a specific configuration andcorresponding zero calibration value. Then, during measurements, theflowmeter determines a density of a currently-measured product, andthereby determines the appropriate zero calibration value automatically.In other words, the meter 502 determines that, based on a given densityof a current product, a particular configuration exists. Then, based onthat density and configuration, the appropriate zero calibration valuemay be selected.

FIG. 6 is. a flowchart 600 illustrating techniques for selecting a zerocalibration value, as described above. In FIG. 6, it is assumed that aparticular flowtube or flowtube type is being used. Thus, if a new ordifferent flowtube were used, the process(es) described below mayrequire repeating to ensure accuracy of the flowmeter in conjunctionwith the new flowtube.

To begin, a user performs a zero calibration for each condition, i.e.,for each operational parameter (602). As in the examples above, thismight include performing zero calibrations for materials havingmultiple, different densities (604), or might include performing zerocalibrations for multiple, different configurations (606), or both. Thezero calibration values are related to their corresponding operationalparameters by the calibration determination and correlation system 230,and these values and their relations to one another (including,possibly, a mathematical relationship between the values) are recordedin the memory 235 (608).

Then, flow of a particular material (in a particular configuration) iscommenced (610). The appropriate zero calibration value is selected bythe calibration selection system 240 (612). For example, the flowmetermay be used to measure a density of the currently-flowing fluid,whereupon the density is correlated with a particular zero calibrationvalue in one or more of the manners described above (614). As anotherexample, the zero calibration value may be selected based on a currentconfiguration/product being used, as input by a user, perhaps by way ofa scrolling, input/output display (616).

Measurements are then performed for some period of time (618). Ifcharacteristics of a new material(s) and/or configuration are to bemeasured (620), then flow of that material (in that configuration) isinitiated (610), and the zero calibration value is adjusted accordingly.Otherwise, the process flow ends (622).

Although density and mechanical configurations are primarily discussedabove as examples of operational parameters that may affect zerocalibration value selection, other factors may affect selection of anappropriate zero calibration value. For example, a zero calibrationvalue may be temperature dependent for wide swings in temperature of,for example, approximately 40 or 50 C or greater. The various proceduresdescribed above can be done with such variations in temperature, sincetemperature also is measured (specifically, using the temperature sensor220).

The techniques described above can be performed, for example, by amanufacturer, during installation of the flowmeter. Alternatively, thetechniques can be performed by a user of the flowmeter, periodically orduring routine measurements. The techniques may be advantageous duringcleaning of the flowmeter, during which cleaning materials may beprocessed through the flowmeter.

Many types of materials may be used in conjunction with theimplementations described herein. For example, the flowmeter zeroingtechniques may be used to measure a density and/or mass flow of liquidssuch as petroleum oil (e.g., Naphtha, natural or distilled) or otherhydrocarbon materials, food and food-related materials, as well ascleaning materials. Similarly, the flowmeter and zeroing techniques maybe used to measure density and/or mass flow of various gasses, includingair, natural gas, and elemental gasses such as Helium. As a result,applications for these and related techniques are far-ranging, includingfood industries, drug production, oil and gas processing, and variousother industries.

Moreover, although the implementations discussed above consider thedirect integration of zeroing techniques with a flowmeter, such asimplementing the techniques in a software module associated with thedigital transmitter 104, it is also possible to determine zerocalibration values independently of the particular transmitter orcontrol system being used. In this case, the zero calibration valuescould be externally subtracted from (or added to) a measurement outputof the digital transmitter 104.

A number of implementations have been described. Nevertheless, it willbe understood that various modifications may be made. Accordingly, otherimplementations are within the scope of the following claims.

1. A method of calibrating a flowmeter having a vibratable flowtube andat least one sensor associated with the vibratable flowtube, the methodcomprising: vibrating the flowtube while a first operational parameterof the flowtube is present; receiving a first sensor signal from thesensor, the sensor signal representative of the vibration of theflowtube while the first operational parameter of the flowtube ispresent; determining a first calibration value based on the first sensorsignal; storing the first calibration value in association with thefirst operational parameter; vibrating the flowtube while a secondoperational parameter of the flowtube is present; receiving a secondsensor signal from the sensor, the second sensor signal representativeof the vibration of the flowtube while the second operational parameterof the flowtube is present; determining a second calibration value basedon the second sensor signal; and storing the second calibration value inassociation with the second operational parameter.
 2. The method ofclaim 1 wherein the first operational parameter comprises a density of afirst material in the flowtube and the second operational parametercomprises a density of a second material in the flowtube.
 3. The methodof claim 1 wherein the first operational parameter comprises a first gasvoid fraction of a material in the flowtube and the second operationalparameter comprises a second gas void fraction material in the flowtube.4. The method of claim 1 wherein the first operational parametercomprises a first configuration of elements associated with the flowtubeand the second operational parameter comprises a second configuration ofelements associated with the flowtube.
 5. The method of claim 1 whereinthe first calibration value is a first zero-flow calibration value andthe second calibration value is a second zero-flow calibration value. 6.A method of calibrating a flowmeter comprising: determining a pluralityof calibration values, the calibration values corresponding tomeasurements of material in a vibratable flowtube determined from asensor signal received from a sensor, the sensor being operable tomeasure vibrations of the flowtube, the flowtube being associated withthe flowmeter; associating each of the calibration values with one of aplurality of operational parameters of the flowmeter, each of theoperational parameters being present during the determining of itscorresponding calibration value; and storing the calibration values inassociation with their respective operational parameters.
 7. The methodof claim 6 wherein determining the plurality of calibration valuescomprises determining a plurality of zero-flow calibration valuescorresponding to the measurements, where the measurements include massflow measurements erroneously indicated by the flowmeter during a timeof substantially zero mass flow through the flowtube.
 8. The method ofclaim 6 further comprising: determining a current operational parameterof the flowmeter; and determining a current calibration value for useduring an obtaining of a mass flow measurement, based on the currentoperational parameter.
 9. The method of claim 8 wherein determining thecurrent operational parameter comprises determining a density of thematerial in the flowtube.
 10. The method of claim 9 wherein determiningthe current calibration value of the flowmeter comprises measuring acurrent density of a current material in the flowtube.
 11. The method ofclaim 9 wherein associating each of the calibration values with one ofthe plurality of operational parameters comprises associating a firstcalibration value with a range of densities.
 12. The method of claim 11wherein determining the current calibration value comprises: measuring acurrent density of a current material in the flowtube; determining thatthe current density falls within the range of densities; and selectingthe first calibration value.
 13. The method of claim 9 whereinassociating each of the calibration values with one of the plurality ofoperational parameters comprises using a mathematical relationshipbetween the calibration values and the plurality of operationalparameters.
 14. The method of claim 13 wherein determining the currentcalibration value comprises: measuring a current density of a currentmaterial in the flowtube; using the current density in conjunction withthe mathematical relationship to determine a current calibration value;and selecting the current calibration value.
 15. The method of claim 8wherein determining the current operational parameter comprisesdetermining a configuration of flow elements associated with theflowtube.
 16. The method of claim 15 wherein determining the currentcalibration value comprises accepting an input from a user, the inputidentifying a current configuration of flow elements.
 17. The method ofclaim 15 wherein determining the current calibration value comprises:measuring a first density of a first material in the flowtube; accessinga correlation between the first density and a first configuration offlow elements; and selecting a first calibration value corresponding tothe first configuration.
 18. The method of claim 8 wherein determiningthe current operational parameter comprises determining a gas voidfraction of the material in the flowtube.
 19. The method of claim 18wherein determining the current calibration value comprises: receivingan input of the gas void fraction from a gas void fraction measurementsystem; and selecting the current calibration value from apre-determined list of associated calibration values and gas voidfractions.
 20. A calibration system comprising: a measurement systemoperable to output measurements of material in a vibratable flowtubedetermined from a sensor signal received from a sensor, the sensor beingoperable to measure vibrations of the flowtube, where the flowtube isassociated with a flowmeter; a calibration system operable to determinecalibration values, each based on a corresponding measurement output bythe measurement system; and a memory operable to store each of thecalibration values in conjunction with an operational parameterassociated with an operation of the flowmeter at a time of thecorresponding measurement.
 21. The system of claim 20 wherein thecalibration system is operable to select a calibration value from thememory, based on a current operational parameter associated with theflowmeter.
 22. The system of claim 20 wherein the measurement includes amass flow rate of the material, and further wherein the calibrationvalues include zero calibration values corresponding toerroneously-detected mass flow measurements of the material during atime of substantially zero flow.
 23. The system of claim 20 wherein theoperational parameter includes a density of the material.
 24. The systemof claim 33 wherein the calibration system is operable to select acurrent calibration value based on a current density of material in theflowtube, as measured by the measurement system.
 25. The system of claim24 wherein the calibration system is operable to select the currentcalibration value by associating the current density with a pre-selectedrange of densities that is stored in the memory in association with thecurrent calibration value.
 26. The system of claim 24 wherein thecalibration system is operable to select the current calibration value,based on the current density and a mathematical relationship between thecalibration values and their corresponding operational parameters. 27.The system of claim 20 wherein the operational parameter includes aconfiguration of flow elements associated with the flowtube.
 28. Thesystem of claim 27 wherein the calibration system is operable to accepta current configuration input by an operator, and to select a currentcalibration value based on the current configuration.
 29. The system ofclaim 27 wherein the calibration system is operable to select a currentcalibration value by determining a current configuration based on acurrently-measured density that was previously associated with thecurrent configuration.
 30. The system of claim 20 wherein theoperational parameter includes a gas void fraction of flow elementswithin the flowtube.
 31. The system of claim 30 wherein the calibrationsystem is operable to receive a current gas void fraction from a gasvoid fraction measurement system, and further operable to select acorresponding current calibration value from the memory.
 32. The systemof claim 20 wherein the measurement system, the calibration system, andthe memory are integrated with the flowmeter.